The effects of fishing on the North and Baltic seas has disturbed the balance in ecosystems.

Predator Becomes Prey

In these seas, the major consumers of fish are porpoises, seals, predatory fish and birds. In southeast Baltic, the herds of seals are small, and whales do not often come to visit here.

The biggest fish eater is the big Atlantic cod; turbot and the short-spined sea scorpion in the coastal zone are the smaller predators. Birds catch a relatiely small part of fish.

In the Baltic Sea, the main herring eater is a cod. Ecologists have discovered that after fishing out a bigger part of predatory fish, in this case it is the cod, the so-called confusion in ecosystem emerges. The affected community spawns with cluperformes, which swallow a lot of cod eggs and grown larvae.

Only Few Survive

Cod spawning period lasts from late March to mid-September. The largest quantities of eggs and larvae are laid in warm saline and oxygenated waters, 25-40 meters deep.

The main food for cod larvae is copepod and cladoreca, also known as water fleas, and their larvae. In 1980-1990, the decrease in salt water brought the edible crustaceans to the lowest level in the Baltic Sea during the whole observation period. The situation has been improving in recent years. The content of edible crustaceans varies in different seasons. In the waters of the island of Bomholm waterhole, which is the most important spawning ground in the Baltic Sea, it is abundant in spring and early summer.

An important condition is that less of cod larvae became predators’ food. Fish researchers from Great Britain, Denmark and Germany found that in different seasons in the North and Baltic seas the cod spawning grounds and clupeiformes’ feeding areas “overlap” for a long time. In addition to crustaceans, herring feed on cod eggs and larvae in cod spawning grounds. Sprat occupies the cod spawning grounds in April and May. After they move to shallower waters to spawn, herring comes back from the coastal zone, which has extremely high small fry gluttony.

How to Overcome the Confusion?

Ecological confusion in the North and Baltic seas has been caused by excess fishing. Having fished out the predatory fish, the species of herring, sprat, anchovy and cod increased. The growth in herring quantity increases cod mortality rate and reduces the size of the cod spawning herd. A massive cod egg and larval mortality rate became evident after summarizing environmental research data of the North and Baltic seas of the period 1975-2010. In the Baltic Sea, this phenomenon is observed from the early nineties.

Since 2006, cod became a subject to taking special measures for protection. Cod fishing was restricted. Recovery in the cod herd is very slow. Stabilization of balance in cod and ferring abundance to the earlier levels can take decades. Aberdeen (UK) North Sea Fish Research Centre researcher Peter Fauchald has suggested restoring the herd of gadiformes. He believes that it is necessary not only to reduce the cod fishing, but to fish out sprat and herring at cod larvae spread areas at the time of spawning. This would reduce the negative impact. So far, this offer causes the fishermen to smile.